As a therapeutic strategy for tumors, it can be expected that a gene of a T cell receptor (TCR) capable of binding to a specific antigen is introduced into any T cell to prepare a T cell targeting the antigen of interest. Based on this strategy, adoptive immunity gene therapies using TCR genes targeting many tumor antigens, for example, WT1, MART1, gp100, CEA, CD19 and mHAG HA-2 antigens have been attempted.
As a novel adoptive immunity gene therapy for tumors, attention has been paid to a gene-modified T cell therapy. The gene-modified T cell therapy comprises introducing a nucleic acid encoding a chimeric antigen receptor (CAR) into a T cell, wherein the CAR has specificity for a surface antigen of a tumor cell and ability to activate a T cell, growing ex vivo the gene-introduced T cell thus obtained, and then transfusing the cell into a patient. This therapy is believed to have a stronger and longer-lasted anti-tumor effect than that of an antibody drug, and therefore the clinical effect thereof is greatly expected.
A representative structure of CAR comprises a single chain variable fragment (scFv) recognizing a surface antigen of a tumor cell, a transmembrane domain, and an intracellular domain of a TCR complex CD3ζ that activates a T cell. A CAR having such a constitution is called a first generation CAR. A gene of a single chain variable fragment portion is isolated from, for example, a hybridoma producing a monoclonal antibody that recognizes a target antigen. A T cell expressing a CAR directly recognizes a surface antigen of a tumor cell independently of the expression of major histocompatibility antigen class I on the tumor cell, and at the same time, activates the T cell, and thereby the CAR-expressing T cell can efficiently kill the tumor cell.
For the purpose of enhancing the ability of a first generation CAR to activate a T cell, a second generation CAR has been developed, wherein an intracellular domain of CD28 which is a costimulatory molecule of a T cell is linked to a first generation CAR. As a further improved version, a third generation CAR has also been developed, wherein an intracellular domain derived from CD137 (4-1BB) or CD134 (OX40) which is a tumor necrosis factor (TNF) receptor superfamily is tandemly linked to a first generation CAR. Thus, many CAR molecules targeting variety of tumor antigens have been reported (Non-Patent Document 1). However, costimulatory molecules used as the intracellular domain for the second generation and third generation CARs which are currently reported are limited. It is known that when linked to a CAR, an intracellular domain derived from every costimulatory molecule of a T cell does not uniformly stimulate a T cell strongly to damage a target tumor cell. For example, it has been reported that a second generation CAR in which an intracellular domain derived from CD137 is linked exhibits a cytotoxic activity only to the same extent as that of a first generation CAR, that is, the intracellular domain has no effect in improving the function of a CAR (Non-Patent Document 2). Therefore, finding of a novel costimulatory molecule that is effective when linked to a CAR has been demanded.
A glucocorticoid-induced tumor necrosis factor receptor (GITR) found as a gene being expressed on a regulatory T cell which is a subset of T cells is a transmembrane protein receptor on a cell surface, and is one member of a TNF receptor superfamily (Non-Patent Document 3). A GITR is shown to exist constitutively on a non-activated T cell. A GITR binds to another transmembrane protein called a GITR ligand (hereinafter, referred to as a GITRL). An agonistic antibody to a GITR is shown to eliminate an immunosuppressive activity of a regulatory T cell, which suggests that a GITRL plays a functional role of controlling the activity of a regulatory T cell via a GITR (see Non-Patent Document 4).